Modified copolyesters and improved multilayer reflective films

ABSTRACT

A multilayered polymer film includes a first set of optical layers and a second set of optical layers. The first set of optical layers is made from a polyester which is often birefringent. The polyesters of the first set of optical layers typically have a composition in which 70–100 mol % of the carboxylate subunits are first carboxylate subunits and 0–30 mo1 % are comonomer carboxylate subunits and 70 to 100 mol % of the glycol subunits are first glycol subunits and 0 to 30 mol % of the glycol subunits are comonomer glycol subunits, where at least 0.5 mol % of the combined carboxylate and glycol subunits are comonomer carboxylate or comonomer glycol subunits. The multilayered polymer film may be used to form, for example, a reflective polarizer or a mirror.

This application is a continuation of U.S. patent application Ser. No.10/676,692, filed Oct. 1, 2003, now U.S. Pat. 6,946,188; which is acontinuation of U.S. patent application Ser. No. 09/996,655, filed Nov.28, 2001, issued as U.S. Pat. No. 6,641,900; which is a continuation ofU.S. patent application Ser. No. 09/232,332, Jan. 15, 1999, now U.S.Pat. No. 6,352,761; which is a continuation-in-part of U.S. applicationSer. No. 09/006,601, filed Jan. 13, 1998 now abandoned.

FIELD OF THE INVENTION

The present invention relates to multilayer optical films having two ormore different sets of layers, each set being formed from a differentpolyester, and to improved polyesters for use in these films.

BACKGROUND OF THE INVENTION

Polymeric films are used in a wide variety of applications. Oneparticular use of polymeric films is in mirrors and polarizers thatreflect light of a given polarization and wavelength range. Suchreflective films are used, for example, in conjunction with backlightsin liquid crystal displays to enhance brightness and reduce glare of thedisplay. A polarizing film may be placed between the user and thebacklight to direct the light towards the user and to polarize thelight; thereby reducing the glare. In addition, a mirror film may beplaced behind the backlight to reflect light towards the user; therebyenhancing brightness. Another use of polarizing films is in articles,such as sunglasses, to reduce light intensity and glare.

One type of polymer that is useful in creating polarizer or mirror filmsis a polyester. One example of a polyester-based polarizer includes astack of polyester layers of differing composition. One configuration ofthis stack of layers includes a first set of birefringent layers and asecond set of layers with an isotropic index of refraction. The secondset of layers alternates with the birefringent layers to form a seriesof interfaces for reflecting light. The polarizer may also include oneor more non-optical layers which, for example, cover at least onesurface of the stack of layers to prevent damage to the stack during orafter processing. There are other configurations that may also be usedin polarizer/mirror films including stacks of layers with two or moredifferent sets of birefringent and/or isotropic layers.

The properties of a given polyester are typically determined by themonomer materials utilized in the preparation of the polyester. Apolyester is often prepared by reactions of one or more differentcarboxylate monomers (e.g., compounds with two or more carboxylic acidor ester functional groups) with one or more different glycol monomers(e.g., compounds with two or more hydroxy functional groups). Each setof polyester layers in the stack typically has a different combinationof monomers to generate the desired properties for each type of layer.There is a need for the development of polyester films for use inpolarizers and mirrors which have improved properties including physicalproperties, optical properties, and lower manufacturing cost.

SUMMARY OF THE INVENTION

Generally, the present invention relates to a multilayered polymer film.One embodiment is a multilayered polymer film which includes a pluralityof first layers and a plurality of second layers. The first layers aremade with a first copolyester which is semicrystalline and birefringent.The first copolyester includes carboxylate subunits and glycol subunitsin which 70 to 100 mol % of the carboxylate subunits are firstcarboxylate subunits, 0 to 30 mol % of the carboxylate subunits arefirst comonomer carboxylate subunits, 70 to 100 mol % of the glycolsubunits are first glycol subunits, and 0 to 30 mol % of the glycolsubunits are first comonomer glycol subunits, and at least 2.5 mol % ofthe combined carboxylate and glycol subunits of the first copolyesterare first comonomer carboxylate subunits, first comonomer glycolsubunits, or a combination thereof. The second layers are made with asecond polymer which has an in-plane birefringence of about 0.04 orless, at 632.8 nm, after the multilayered polymer film has been formed.

Another embodiment is a multilayered polymer film having a plurality offirst layers and a plurality of second layers. The first layers are madewith a first copolyester which is semicrystalline and birefringent. Thefirst copolyester includes carboxylate subunits and glycol subunits inwhich 70 to 100 mol % of the carboxylate subunits are first carboxylatesubunits, 0 to 30 mol % of the carboxylate subunits are first comonomercarboxylate subunits, 70 to 100 mol % of the glycol subunits are firstglycol subunits, 0 to 30 mol % of the glycol subunits are firstcomonomer glycol subunits, and at least 0.5 mol % of the combinedcarboxylate and glycol subunits of the first copolyester are firstcomonomer carboxylate subunits, first comonomer glycol subunits, or acombination thereof. The first copolyester has in-plane indices ofrefraction which are 1.83 or less and which differ by 0.2 or greaterwhen measured with 632.8 nm light. The second layers are made with asecond polymer which has an in-plane birefringence of about 0.04 or lessat 632.8 nm after the multilayered polymer film has been formed.

A further embodiment is a multilayered polymer film which has aplurality of first layers and a plurality of second layers. The firstlayers are made with a first copolyester which is semicrystalline andbirefringent. The first copolyester has carboxylate subunits and glycolsubunits in which 70 to 100 mol % of the carboxylate subunits are firstcarboxylate subunits, 0 to 30 mol % of the carboxylate subunits arefirst comonomer carboxylate subunits, 70 to 100 mol % of the glycolsubunits are first glycol subunits, 0 to 30 mol % of the glycol subunitsare first comonomer glycol subunits, and at least 0.5 mol % of thecombined carboxylate and glycol subunits of the first copolyester arefirst comonomer carboxylate subunits, first comonomer glycol subunits,or a combination thereof. The second layers are made with a secondpolymer having an in-plane birefringence of about 0.04 or less at 632.8nm after the multilayered polymer film has been formed. The multilayeredpolymer film is formed by drawing the first and second layers in atleast one draw direction to a particular draw ratio. After drawing thefirst and second layers, the first layers of the multilayered polymerfilm have an index of refraction in the draw direction which, at 632.8nm, is at least 0.02 units less than an index of refraction in the drawdirection of a similarly constructed polyethylene naphthalate layerwhich has a same in-plane birefringence and draw ratio.

Yet another embodiment is a multilayered polymer film which includes aplurality of first layers and a plurality of second layers. The firstlayers are made with a first copolyester which is semicrystalline andbirefringent. The first copolyester has carboxylate subunits and glycolsubunits in which 70 to 100 mol % of the carboxylate subunits are firstcarboxylate subunits, 70 to 99 mol % of the glycol subunits are firstglycol subunits, and 1 to 30 mol % of the glycol subunits are firstcomonomer glycol subunits. The second layers are made with a secondpolymer which has an in-plane birefringence of about 0.04 or less, at632.8 nm, after the multilayered polymer film has been formed.

Another embodiment is a multilayered polymeric film which has aplurality of first layers and a plurality of second layers. The firstlayers are made with a first copolyester which is semicrystalline andbirefringent. The second layers are made with a second copolyester whichhas an in-plane birefringence of about 0.04 or less, at 632.8 nm, afterthe multilayered polymer film has been formed. The second copolyesterincludes carboxylate subunits and glycol subunits in which 0.01 to 2.5mol % of the combined carboxylate and glycol subunits are derived fromcompounds with three or more carboxylate or ester functional groups,compounds with three or more hydroxy functional groups, or a combinationthereof.

A further embodiment is a multilayer polymer film which includes aplurality of first layers and a plurality of second layers. The firstlayers are made with a first copolyester which is semicrystalline andbirefringent. The second layers are made with a second copolyester ofpolyethylene naphthalate. The second copolyester includes glycolsubunits and carboxylate subunits in which the glycol subunits are 70 to100 mol % ethylene or butylene subunits and about 0 to 30 mol %comonomer glycol subunits derived from one or more of 1,6-hexanediol,trimethylol propane, or neopentyl glycol, and the carboxylate subunitsare 20 to 100 mol % naphthalate subunits, 0 to 80 mol % terephthalate orisophthalate subunits or mixtures thereof, and 0 to 30 mol % ofcomonomer carboxylate subunits derived from phthalic acid,t-butyl-isophthalic acid, lower alkyl esters of these acids, or acombination thereof, and at least 0.5 mol % of the combined carboxylateand glycol subunits of the second copolyester are comonomer carboxylatesubunits, comonomer glycol subunits, or a combination thereof.

Another embodiment is a polymer which is a copolyester having anintrinsic viscosity of about 0.4 dL/g or greater as measured in a 60/40wt. % mixture of phenol/o-dichlorobenzene. The polymer includes glycolsubunits and carboxylate subunits in which the glycol subunits are 70 to99 mol % ethylene or butylene subunits and 1 to 30 mol % comonomerglycol subunits derived from 1,6-hexanediol, and the carboxylatesubunits are 5 to 99 mol % naphthalate subunits, 1 to 95 mol %terephthalate or isophthalate subunits or a combination thereof, and 0to 30 mol % of comonomer carboxylate subunits derived from phthalicacid, t-butyl-isophthalic acid, lower alkyl esters of these acids, or acombination thereof; and at least 0.01 to 2.5 mol % of the combinedcarboxylate and glycol subunits of the copolyester are derived fromcompounds having three or more carboxylate, ester, or hydroxy functionalgroups.

One other embodiment is a multilayer polymer film which includes aplurality of birefringent first layers and a plurality of second layers.The first layers are made with a first copolyester having naphthalatesubunits. The second layers are made with a second copolyester which hasan intrinsic viscosity of 0.4 to 0.5 dL/g and contains 0.01 to 5 mol %comonomer subunits which are derived from compounds having three or morecarboxylate, ester, or hydroxy functional groups. The multilayer polymerfilm also includes one or more non-optical layers that have an intrinsicviscosity of 0.5 dL/g or greater.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of one embodiment of a multilayeredpolymer film according to the present invention;

FIG. 2 is a cross-sectional view of another embodiment of a multilayeredpolymer film according to the present invention;

FIGS. 3A and 3B are graphs illustrating the decrease in glass transitiontemperature (FIG. 3A) and freezing temperature (FIG. 3B) with theaddition of terephthalate (using dimethyl terephthalate (DMT)) andisophthalate (using dimethyl isophthalate (DMI)) subunits topolyethylene naphthalate (PEN) which is derived from dimethylnaphthalene dicarboxylate;

FIG. 4 is a graph of the average in-plane birefringence of coPENmodified with terephthalate and isophthalate subunits and oriented atrelatively low temperatures;

FIG. 5 is a graph of the thermal stability of coPEN containingterephthalate and isophthalate subunits;

FIG. 6 is a graph illustrating the reduction in in-plane birefringence,at 632.8 nm, of a coPEN by the addition of comonomer subunits; and

FIG. 7 is a graph illustrating the dependence of in-plane birefringence,at 632.8 nm, on molecular weight.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention relates to multilayered polymer films for opticalapplications and the use of comonomer subunits to enhance the propertiesof the polymer films and, in particular, to enhance the properties ofpolymer films made from polyesters having naphthalate subunits,including, for example, copolymers of polyethylene naphthalate.

FIG. 1 shows a multilayered polymer film 10 which may be used, forexample, as an optical polarizer or mirror. The film 10 includes one ormore first optical layers 12, one or more second optical layers 14, andone or more non-optical layers 18. The first optical layers 12 arepreferably birefringent polymer layers which are uniaxially-orbiaxially-oriented. The second optical layers 14 may also be polymerlayers which are birefringent and uniaxially- or biaxially-oriented.More typically, however, the second optical layers 14 have an isotropicindex of refraction which is different from at least one of the indicesof refraction of the first optical layers 12 after orientation. Themethods of manufacture and use, as well as design considerations for themultilayered polymer films 10 are described in detail in U.S. Pat. No.5,882,774 entitled “Multilayered Optical Film” and U.S. patentapplication Ser. No. 09/006,288 entitled “Process for Making MultilayerOptical Film.” Although, the present invention will be primarilyexemplified by films 10 with second optical layers 14 which have anisotropic index of refraction, the principles and examples describedherein may be applied to multilayered polymer films with second opticallayers 14 that are birefringent, as described in U.S. Pat. No.6,113,811, entitled “Optical Film and Process for the ManufactureThereof.”

Additional sets of optical layers, similar to the first and secondoptical layers 12, 14, may also be used in the multilayered polymer film10. The design principles disclosed herein for the sets of first andsecond optical layers may be applied to any additional sets of opticallayers. Furthermore, it will be appreciated that, although only a singlestack 16 is illustrated in FIG. 1, the multilayered polymer film 10 maybe made from multiple stacks that are subsequently combined to form thefilm 10.

The optical layers 12, 14 and, optionally, one or more of thenon-optical layers 18 are typically placed one on top of the other toform a stack 16 of layers. Usually the optical layers 12, 14 arearranged as alternating pairs, as shown in FIG. 1, to form a series ofinterfaces between layers with different optical properties. The opticallayers 12, 14 are typically less than 1 μm thick, although thickerlayers may be used. Furthermore, although FIG. 1 shows only six opticallayers 12, 14, many multilayered polymer films 10 have a large number ofoptical layers. Typical multilayered polymer films have about 2 to 5000optical layers, preferably about 25 to 2000 optical layers, morepreferably about 50 to 1500 optical layers, and most preferably about 75to 1000 optical layers.

The non-optical layers 18 are polymer layers that are disposed within(see FIG. 2) and/or over (see FIG. 1) the stack 16 to protect theoptical layers 12, 14 from damage, to aid in the co-extrusionprocessing, and/or to enhance post-processing mechanical properties. Thenon-optical layers 18 are often thicker than the optical layers 12, 14.The thickness of the non-optical layers 18 is usually at least twotimes, preferably at least four times, and more preferably at least tentimes, the thickness of the individual optical layers 12, 14. Thethickness of the non-optical layers 18 may be varied to make amultilayer polymer film 10 having a particular thickness. Typically, oneor more of the non-optical layers 18 are placed so that at least aportion of the light to be transmitted, polarized, and/or reflected bythe optical layers 12, 14, also travels through the non-optical layers(i.e., the non-optical layers are placed in the path of light whichtravels through or is reflected by the optical layers 12, 14).

The optical layers 12, 14 and the non-optical layers 18 of themultilayered polymer film 10 are typically composed of polymers such aspolyesters. Polyesters include carboxylate and glycol subunits and aregenerated by reactions of carboxylate monomer molecules with glycolmonomer molecules. Each carboxylate monomer molecule has two or morecarboxylic acid or ester functional groups and each glycol monomermolecule has two or more hydroxy functional groups. The carboxylatemonomer molecules may all be the same or there may be two or moredifferent types of molecules. The same applies to the glycol monomermolecules. The term “polymer” will be understood to include bothpolymers and copolymers, as well as polymers or copolymers which may beformed in a miscible blend, for example, by coextrusion or by reaction,including, for example, transesterification.

The properties of a polymer layer or film vary with the particularchoice of monomer molecules. One example of a polyester useful inmultilayered optical films is polyethylene naphthalate (PEN) which canbe made, for example, by reactions of naphthalene dicarboxylic acid withethylene glycol.

Suitable carboxylate monomer molecules for use in forming thecarboxylate subunits of the polyester layers include, for example,2,6-naphthalene dicarboxylic acid and isomers thereof; terephthalicacid; isophthalic acid; phthalic acid; azelaic acid; adipic acid;sebacic acid; norbornene dicarboxylic acid; bi-cyclooctane dicarboxylicacid; 1,6-cyclohexane dicarboxylic acid and isomers thereof; t-butylisophthalic acid, tri-mellitic acid, sodium sulfonated isophthalic acid;2,2′-biphenyl dicarboxylic acid and isomers thereof; and lower alkylesters of these acids, such as methyl or ethyl esters. The term “loweralkyl” refers, in this context, to C1–C10 straight-chained or branchedalkyl groups. Also included within the term “polyester” arepolycarbonates which are derived from the reaction of glycol monomermolecules with esters of carbonic acid.

Suitable glycol monomer molecules for use in forming glycol subunits ofthe polyester layers include ethylene glycol; propylene glycol;1,4-butanediol and isomers thereof; 1,6-hexanediol; neopentyl glycol;polyethylene glycol; diethylene glycol; tricyclodecanediol;1,4-cyclohexanedimethanol and isomers thereof; norbornanediol;bicyclo-octanediol; trimethylol propane; pentaerythritol;1,4-benzenedimethanol and isomers thereof; bisphenol A; 1,8-dihydroxybiphenyl and isomers thereof; and 1,3-bis(2-hydroxyethoxy)benzene.

Non-polyester polymers are also useful in creating polarizer or mirrorfilms. For example, layers made from a polyester such as polyethylenenaphthalate may be combined with layers made from an acrylic polymer toform a highly reflective mirror film. In addition, polyether imides mayalso be used with polyesters, such as PEN and coPEN, to generate amultilayered optical film. Other polyester/non-polyester combinations,such as polybutylene terephthalate and polyvinyl chloride, may also beused.

The first optical layers 12 are typically orientable polymer films, suchas polyester films, which may be made birefringent by, for example,stretching the first optical layers 12 in a desired direction ordirections. The term “birefringent” means that the indices of refractionin orthogonal x, y, and z directions are not all the same. For films orlayers in a film, a convenient choice of x, y, and z axes is shown inFIG. 1 in which the x and y axes correspond to the length and width ofthe film or layer and the z axis corresponds to the thickness of thelayer or film. In the embodiment illustrated in FIG. 1, the film 10 hasseveral optical layers 12, 14 which are stacked one on top of the otherin the z-direction.

The first optical layers 12 may be uniaxially-oriented, for example, bystretching in a single direction. A second orthogonal direction may beallowed to neck into some value less than its original length. In oneembodiment, the direction of stretching substantially corresponds toeither the x or y axis shown in FIG. 1. However, other directions may bechosen. A birefringent, uniaxially-oriented layer typically exhibits adifference between the transmission and/or reflection of incident lightrays having a plane of polarization parallel to the oriented direction(i.e., stretch direction) and light rays having a plane of polarizationparallel to a transverse direction (i.e., a direction orthogonal to thestretch direction). For example, when an orientable polyester film isstretched along the x axis, the typical result is that n_(x)≠n_(y),where n_(x) and n_(y) are the indices of refraction for light polarizedin a plane parallel to the “x” and “y” axes, respectively. The degree ofalteration in the index of refraction along the stretch direction willdepend on factors such as the amount of stretching, the stretch rate,the temperature of the film during stretching, the thickness of thefilm, the variation in the film thickness, and the composition of thefilm. Typically, the first optical layers 12 have an in-planebirefringence (the absolute value of n_(x)−n_(y)) after orientation of0.04 or greater at 632.8 nm, preferably about 0.1 or greater, and morepreferably about 0.2 or greater. All birefringence and index ofrefraction values are reported for 632.8 nm light unless otherwiseindicated.

Polyethylene naphthalate (PEN) is an example of a useful material forforming the first optical layers 12 because it is highly birefringentafter stretching. The refractive index of PEN for 632.8 nm lightpolarized in a plane parallel to the stretch direction increases fromabout 1.62 to as high as about 1.87. Within the visible spectrum, PENexhibits a birefringence of 0.20 to 0.40 over a wavelength range of400–700 nm for a typical high orientation stretch (e.g., a materialstretched to five or more times its original dimension at a temperatureof 130° C. and an initial strain rate of 20%/min).

The birefringence of a material can be increased by increasing themolecular orientation. Many birefringent materials are crystalline orsemicrystalline. The term “crystalline” will be used herein to refer toboth crystalline and semicrystalline materials. PEN and othercrystalline polyesters, such as polybutylene naphthalate (PBN),polyethylene terephthalate (PET) and polybutylene terephthalate (PBT)are examples of crystalline materials useful in the construction ofbirefringent film layers, such as is often the case for the firstoptical layers 12. In addition, some copolymers of PEN, PBN, PET, andPBT are also crystalline or semicrystalline. The addition of a comonomerto PEN, PBN, PET, or PBT may enhance other properties of the materialincluding, for example, adhesion to the second optical layers 14 or thenon-optical layers 18 and/or the lowering of the working temperature(i.e., the temperature for extrusion and/or stretching the film).

In some embodiments, the first optical layers 12 are made from asemicrystalline, birefringent copolyester which includes 70 to 99 mol %of a first carboxylate subunit and 1 to 30 mol %, and preferably 5 to 15mol %, of comonomer carboxylate subunits. The comonomer carboxylatesubunits may be one or more of the subunits indicated hereinabove.Preferred first carboxylate subunits include naphthalate andterephthalate.

If the polyester material of the first optical layers 12 contains morethan one type of carboxylate subunit, then the polyester may be a blockcopolyester to enhance adhesion to other layers (e.g., the secondoptical layers 14 or non-optical layers 18) made from block copolymershaving similar blocks. Random copolyesters may also be used.

In other embodiments, the first optical layers 12 are made from asemicrystalline, birefringent copolyester which includes 70 to 99 mol %of a first glycol subunit and 1 to 30 mol %, and preferably 5 to 30 mol% of comonomer glycol subunits. The comonomer glycol subunits may be oneor more of the subunits indicated hereinabove. Preferred first glycolsubunits are derived from C2–C8 diols. More preferred first glycolsubunits are derived from ethylene glycol or 1,4-butanediol.

Yet other embodiments include first optical layers 12 where both of thecarboxylate and glycol subunits include comonomer subunits. For theseembodiments, typically at least 0.5 mol %, and preferably at least 2.5mol %, of the combined carboxylate and glycol subunits are comonomercarboxylate subunits, comonomer glycol subunits, or a combinationthereof.

With the increasing addition of comonomer carboxylate and/or glycolsubunits, the index of refraction in the orientation direction,typically the largest index of refraction, often decreases. Based onsuch an observation, this might lead to a conclusion that thebirefringence of the first optical layers will be proportionatelyaffected. However, it has been found that the index of refraction in thetransverse direction also decreases with the addition of comonomersubunits. This results in substantial maintenance of the birefringence.

For example, the addition of 3 mol % isophthalate subunits topolyethylene naphthalate reduces the melt processing temperature fromabout 280° C. to about 265° C. with only a 0.02 unit loss inbirefringence. FIGS. 3A and 3B illustrate the reduction in glasstransition temperature and freezing point temperature for the additionof 3 to 9 mol % isophthalate (derived from dimethyl isophthalate (DMI))or terephthalate (derived from dimethyl terephthalate (DMT)) subunits.In general, the reduction in freezing point is typically greater thanthe change in the glass transition temperature for a given amount ofsubstituted subunits. FIG. 4 illustrates the average birefringence of alow melt point coPEN having 0 to 9 mol % terephthalate and isophthalatesubunits. This low melt point coPEN typically has better adhesion tosecond optical layers made from a coPEN which contains terephthalateand/or isophthalate subunits due to the presence of common monomersubunits.

In many cases, a multilayered polymer film 10 may be formed using firstoptical layers 12 that are made from a coPEN which has the same in-planebirefringence for a given draw ratio (i.e., the ratio of the length ofthe film in the stretch direction after stretching and beforestretching) as a similar multilayered polymer film formed using PEN forthe first optical layers. The matching of birefringence values may beaccomplished by the adjustment of processing parameters, such as theprocessing or stretch temperatures. Often coPEN optical layers have anindex of refraction in the draw direction which is at least 0.02 unitsless than the index of refraction of the PEN optical layers in the drawdirection. The birefringence is maintained because there is a decreasein the index of refraction in the non-draw direction.

In some preferred embodiments of the multilayered polymer films, thefirst optical layers are made from a coPEN which has in-plane indices ofrefraction (i.e., n_(x) and n_(y)) that are 1.83 or less, and preferably1.80 or less, and which differ (i.e., |n_(x)−n_(y)|) by 0.15 units ormore, and preferably 0.2 units or more, when measured using 632.8 nmlight. PEN often has an in-plane index of refraction that is 1.84 orhigher and the difference between the in-plane indices of refraction isabout 0.22 to 0.24 or more when measured using 632.8 nm light. Thein-plane refractive index differences, or birefringence, of the firstoptical layers, whether they be PEN or coPEN, may be reduced to lessthan 0.2 to improve properties, such as interlayer adhesion. Similarcomparisons between suitable coPBN and coPET polymers for the firstlayers can be made with PBN and PET.

The second optical layers 14 may be made from a variety of polymers.Examples of suitable polymers include vinyl polymers and copolymers madefrom monomers such as vinyl naphthalenes, styrene, maleic anhydride,acrylates, and methacrylates. Examples of such polymers includepolyacrylates, polymethacrylates, such as poly(methyl methacrylate)(PMMA), and isotactic or syndiotactic polystyrene. Other polymersinclude condensation polymers such as polysulfones, polyamides,polyurethanes, polyamic acids, and polyimides. In addition, the secondoptical layers 14 may be formed from polymers and copolymers such aspolyesters and polycarbonates. The second optical layers 14 will beexemplified below by copolymers of polyesters. However, it will beunderstood that the other polymers described above may also be used. Thesame considerations with respect to optical properties for thecopolyesters, as described below, will also typically be applicable forthe other polymers and copolymers.

In some embodiments, the second optical layers 14 are uniaxially orbiaxially orientable. However, more typically the second optical layers14 are not oriented under the processing conditions used to orient thefirst optical layers 12. These second optical layers 14 typically retaina relatively isotropic index of refraction, even when stretched.Preferably, the second optical layers have a birefringence of less thanabout 0.04, and more preferably less than about 0.02 at 632.8 nm.

Examples of suitable materials for the second optical layers 14 arecopolymers of PEN, PBN, PET, or PBT. Typically, these copolymers includecarboxylate subunits which are 20 to 100 mol % second carboxylatesubunits, such as naphthalate (for coPEN or coPBN) or terephthalate (forcoPET or coPBT) subunits, and 0 to 80 mol % second comonomer carboxylatesubunits. The copolymers also include glycol subunits which are 40 to100 mol % second glycol subunits, such as ethylene (for coPEN or coPET)or butylene (for coPBN or coPBT), and 0 to 60 mol % second comonomerglycol subunits. At least about 10 mol % of the combined carboxylate andglycol subunits are second comonomer carboxylate or glycol subunits.

One example of a polyester for use in second optical layers 14 is a lowcost coPEN. One currently used coPEN has carboxylate subunits which areabout 70 mol % naphthalate and about 30 mol % isophthalate. Low costcoPEN replaces some or all of the isophthalate subunits withterephthalate subunits. The cost of this polymer is reduced as dimethylisophthalate, the typical source for the isophthalate subunits,currently costs considerably more than dimethyl terephthalate, a sourcefor the terephthalate subunits. Furthermore, coPEN with terephthalatesubunits tends to have greater thermal stability than coPEN withisophthalate subunits, as illustrated in FIG. 5.

However, substitution of terephthalate for isophthalate may increase thebirefringence of the coPEN layer; so a combination of terephthalate andisophthalate may be desired. Low cost coPEN typically has carboxylatesubunits in which 20 to 80 mol % of the carboxylate subunits arenaphthalate, 10 to 60 mol % are terephthalate, and 0 to 50 mol % areisophthalate subunits. Preferably, 20 to 60% mol % of the carboxylatesubunits are terephthalate and 0 to 20 mol % are isophthalate. Morepreferably, 50 to 70 mol % of the carboxylate subunits are naphthalate,20 to 50 mol % are terephthalate, and 0 to 10 mol % are isophthalatesubunits.

Because coPENs may be slightly birefringent and orient when stretched,it may be desirable to produce a polyester composition for use withsecond optical layers 14 in which this birefringence is reduced. Lowbirefringent coPENs may be synthesized by the addition of comonomermaterials. Examples of suitable birefringent-reducing comonomermaterials for use as diol subunits are derived from 1,6-hexanediol,trimethylol propane, and neopentyl glycol. Examples of suitablebirefringent-reducing comonomer materials for use as carboxylatesubunits are derived from t-butyl-isophthalic acid, phthalic acid, andlower alkyl esters thereof. FIG. 6 is a graph illustrating the reductionin birefringence of coPEN by addition of these materials. This reductionmay be 0.07 or higher at 632.8 nm when the second optical layers 14 havebeen drawn under high strain conditions (e.g., at a draw ratio at 5:1 orgreater) or under a low draw temperature. The addition of comonomers inthe coPEN also increases the normal angle gain of the optical polarizer.Normal angle gain is a measure of the increase in light emitted from anLCD when the reflective polarizer is used in combination with anabsorbing polymer.

Preferred birefringent-reducing comonomer materials are derived fromt-butyl-isophthalic acid, lower alkyl esters thereof, and1,6-hexanediol. Other preferred comonomer materials are trimethylolpropane and pentaerythritol which may also act as branching agents. Thecomonomers may be distributed randomly in the coPEN polyester or theymay form one or more blocks in a block copolymer.

Examples of low birefringent coPEN include glycol subunits which arederived from 70–100 mol % C2–C4 diols and about 0–30 mol % comonomerdiol subunits derived from 1,6-hexanediol or isomers thereof,trimethylol propane, or neopentyl glycol and carboxylate subunits whichare 20 to 100 mol % naphthalate, 0 to 80 mol % terephthalate orisophthalate subunits or mixtures thereof, and 0 to 30 mol % ofcomonomer carboxylate subunits derived from phthalic acid,t-butyl-isophthalic acid, or lower alkyl esters thereof. Furthermore,the low birefringence coPEN has at least 0.5 to 5 mol % of the combinedcarboxylate and glycol subunits which are comonomer carboxylate orglycol subunits.

The addition of comonomer subunits derived from compounds with three ormore carboxylate, ester, or hydroxy functionalities may also decreasethe birefringence of the copolyester of the second layers. Thesecompounds act as branching agents to form branches or crosslinks withother polymer molecules. In some embodiments of the invention, thecopolyester of the second layer includes 0.01 to 5 mol %, preferably 0.1to 2.5 mol %, of these branching agents.

One particular polymer has glycol subunits that are derived from 70 to99 mol % C2–C4 diols and about 1 to 30 mol % comonomer subunits derivedfrom 1,6-hexanediol and carboxylate subunits that are 5 to 99 mol %naphthalate, 1 to 95 mol % terephthalate, isophthalate, or mixturesthereof, and 0 to 30 mol % comonomer carboxylate subunits derived fromone or more of phthalic acid, t-butyl-isophthalic acid, or lower alkylesters thereof. In addition, at least 0.01 to 2.5 mol % of the combinedcarboxylate and glycol subunits of this copolyester are branchingagents.

Because birefringence typically decreases with molecular weight, anotheruseful polyester is a low molecular weight coPEN. The low molecularweight coPEN has an intrinsic viscosity of 0.4 to 0.5 dL/g. Theintrinsic viscosity of the polymer is retained by the addition ofbetween about 0.5 to 5 mol % of monomers having three or morecarboxylate, ester, and/or hydroxy groups. These monomers often act asbranching agents. The molecular weight of the polymer is established byending the polymerization at a specified melt viscosity determined by,for example, the power draw on a reactor agitator, agitator speed, andmelt temperature. Typically, non-optical layers having an intrinsicviscosity of 0.5 dL/g or greater are used with this low molecular weightcoPEN to provide structural support.

Suitable branching monomers for use in increasing the melt viscosity ofa low molecular weight coPEN include alcohols with more than two hydroxyfunctionalities, as well as carboxylic acids with more than twocarboxylic acid functionalities and lower alkyl esters thereof. Examplesof suitable branching monomers include trimethylol propane,pentaerythritol, and trimellitic acid. FIG. 7 illustrates the decreasein birefringence with decrease in molecular weight (as measured bydecrease in intrinsic viscosity).

Another type of useful copolyester includes cyclohexane dicarboxylatesubunits. These copolyesters are especially useful as low refractiveindex polymers due to their viscoelastic properties which enable stablemultilayer coextrusion with polyethylene naphthalate in the firstoptical layers 12. In contrast, some other aliphatic copolyesters withlow refractive indices do not have the rheological properties necessaryto provide stable melt flow when coextruded in a multilayer meltmanifold with polyethylene naphthalate. Cyclohexane dicarboxylate alsomay provide improved thermal stability over other low refractive indexcopolyesters during coextrusion.

Tertiary-butyl isophthalate is a preferred carboxylate subunit for usewith cyclohexane dicarboxylate in effectively improving the glasstransition temperature and modulus of the copolyester withoutsubstantially increasing the refractive indices. The addition oftertiary-butyl isophthalate enables copolyesters of cyclohexanedicarboxylate to have glass transition temperatures above roomtemperature with refractive indices as low as 1.51 at 632.8 nm.Utilizing branching monomers such as trimethylol propane enables highviscosity polymers to be synthesized from these monomers without theneed for large amounts of catalyst or long reaction times, whichimproves color and clarity of the polymer. Thus, non-birefringentcopolyesters with low refractive indices may be produced withcyclohexane dicarboxylate and tertiary-butyl isophthalate providing thecarboxylate subunits, and ethylene glycol and trimethylol propaneproviding the glycol subunits. These copolyesters are useful for makingmultilayer optical films which retain their physical properties at roomtemperature. Copolyesters made using naphthalene dicarboxylate andcyclohexane dicarboxylate as carboxylates can be coextruded withpolyethylene naphthalate to form a multilayered polymer film with goodinterlayer adhesion.

One embodiment of the invention includes second optical layers made froma polyester with carboxylate subunits derived from cyclohexanedicarboxylate. Preferably, the polyester has carboxylate subunitsderived from 5 to 95 mol % dimethyl cyclohexane dicarboxylate and 5 to95 mol % dimethyl tertiary-butyl isophthalate and glycol subunitsderived from 85 to 99.99 mol % C2–C4 diols and 0.01 to 5 mol %trimethylol propane. More preferably, the polyester has carboxylatesubunits derived from 50 to 85 mol % dimethyl cyclohexane dicarboxylateand 15 to 50 mol % dimethyl tertiary-butyl isophthalate and glycolsubunits derived from 98 to 99.99 mol % C2–C4 diols and 0.01 to 2 mol %trimethylol propane.

The non-optical layers 18 may also be made from copolyesters similar tothe second optical layers 14, using similar materials and similaramounts of each material. In addition, other polymers may also be used,as described above with respect to the second optical layers 14. It hasbeen found that the use of coPEN (i.e., a copolymer of PEN) or othercopolymer material for skin layers (as seen in FIG. 1) reduces thesplittiness (i.e., the breaking apart of a film due to strain-inducedcrystallinity and alignment of a majority of the polymer molecules inthe direction of orientation) of the multilayered polymer film, becausethe coPEN of the skin layers orients very little when stretched underthe conditions used to orient the first optical layers 12.

Preferably, the polyesters of the first optical layers 12, the secondoptical layers 14, and the non-optical layers 18 are chosen to havesimilar rheological properties (e.g., melt viscosities) so that they canbe co-extruded. Typically, the second optical layers 14 and thenon-optical layers 18 have a glass transition temperature, T_(g), thatis either below or no greater than about 40° C. above the glasstransition temperature of the first optical layers 12. Preferably, theglass transition temperature of the second optical layers 14 and thenon-optical layers 18 is below the glass transition temperature of thefirst optical layers 12.

A polarizer may be made by combining a uniaxially-oriented first opticallayer 12 with a second optical layer 14 having an isotropic index ofrefraction that is approximately equal to one of the in-plane indices ofthe oriented layer. Alternatively, both optical layers 12,14 are formedfrom birefringent polymers and are oriented in a multiple draw processso that the indices of refraction in a single in-plane direction areapproximately equal. The interface between the two optical layers 12,14,in either case, forms a light reflection plane. Light polarized in aplane parallel to the direction in which the indices of refraction ofthe two layers are approximately equal will be substantiallytransmitted. Light polarized in a plane parallel to the direction inwhich the two layers have different indices will be at least partiallyreflected. The reflectivity can be increased by increasing the number oflayers or by increasing the difference in the indices of refractionbetween the first and second layers 12, 14.

Typically, the highest reflectivity for a particular interface occurs ata wavelength corresponding to twice the combined optical thickness ofthe pair of optical layers 12, 14 which form the interface. The opticalthickness of the two layers is n₁d₁+n₂d₂ where n₁, n₂ are the indices ofrefraction of the two layers and d₁, d₂ are the thicknesses of thelayers. The layers 12, 14 may each be a quarter wavelength thick or thelayers 12, 14 may have different optical thicknesses, so long as the sumof the optical thicknesses is half of a wavelength (or a multiplethereof). A film having a plurality of layers may include layers withdifferent optical thicknesses to increase the reflectivity of the filmover a range of wavelengths. For example, a film may include pairs oflayers which are individually tuned to achieve optimal reflection oflight having particular wavelengths.

Alternatively, the first optical layers 12 may be biaxially-oriented bystretching in two different directions. The stretching of optical layers12 in the two directions may result in a net symmetrical or asymmetricalstretch in the two chosen orthogonal axes.

One example of the formation of a mirror is the combination of abiaxially-oriented optical layer 22 with a second optical layer 24having indices of refraction which differ from both the in-plane indicesof the biaxially-oriented layer. The mirror operates by reflecting lighthaving either polarization because of the index of refraction mismatchbetween the two optical layers 12, 14. Mirrors may also be made using acombination of uniaxially-oriented layers with in-plane indices ofrefraction which differ significantly. In another embodiment, the firstoptical layers 12 are not birefringent and a mirror is formed bycombining first and second optical layers 12, 14 which havesignificantly different indices of refraction. Reflection occurs withoutorientation of the layers. There are other methods and combinations oflayers that are known for producing both mirrors and polarizers andwhich may be used. Those particular combinations discussed above aremerely exemplary.

The second optical layers 14 may be prepared with a variety of opticalproperties depending, at least in part, on the desired operation of thefilm 10. In one embodiment, the second optical layers 14 are made of apolymer material that does not appreciably optically orient whenstretched under conditions which are used to orient the first opticallayers 12. Such layers are particularly useful in the formation ofreflective polarizing films, because they allow the formation of a stack16 of layers by, for example, coextrusion, which can then be stretchedto orient the first optical layers 12 while the second optical layers 14remain relatively isotropic. Typically, the index of refraction of thesecond optical layers 14 is approximately equal to one of the indices ofthe oriented first optical layers 12 to allow transmission of light witha polarization in a plane parallel to the direction of the matchedindices. Preferably, the two approximately equal indices of refractiondiffer by about 0.05 or less, and more preferably by about 0.02 or less,at 632.8 nm. In one embodiment, the index of refraction of the secondoptical layers 14 is approximately equal to the index of refraction ofthe first optical layers 12 prior to stretching.

In other embodiments, the second optical layers 14 are orientable. Insome cases, the second optical layers 14 have one in-plane index ofrefraction that is substantially the same as the corresponding index ofrefraction of the first optical layers 12 after orientation of the twosets of layers 12, 14, while the other in-plane index of refraction issubstantially different than that of the first optical layers 12. Inother cases, particularly for mirror applications, both in-plane indicesof refraction of the optical layers 12, 14 are substantially differentafter orientation.

Referring again to FIGS. 1 and 2, one or more of the non-optical layers18 may be formed as a skin layer over at least one surface of stack 16as illustrated in FIG. 1, to, for example, protect the optical layers12, 14 from physical damage during processing and/or afterwards. Inaddition, one or more of non-optical layers 18 may be formed within thestack 16 of layers, as illustrated in FIG. 2, to, for example, providegreater mechanical strength to the stack or to protect the stack duringprocessing.

The non-optical layers 18 ideally do not significantly participate inthe determination of optical properties of the multilayered polymer film10, at least across the wavelength region of interest. The non-opticallayers 18 are typically not birefringent or orientable but in some casesthis may not be true. Typically, when the non-optical layers 18 are usedas skin layers there will be at least some surface reflection. If themultilayered polymer film 10 is to be a polarizer, the non-opticallayers preferably have an index of refraction which is relatively low.This decreases the amount of surface reflection. If the multilayeredpolymer film 10 is to be a mirror, the non-optical layers 18 preferablyhave an index of refraction which is high, to increase the reflection oflight.

When the non-optical layers 18 are found within the stack 16, there willtypically be at least some polarization or reflection of light by thenon-optical layers 18 in combination with the optical layers 12, 14adjacent to the non-optical layers 18. Typically, however, thenon-optical layers 18 have a thickness which dictates that lightreflected by the non-optical layers 18 within the stack 16 has awavelength outside the region of interest, for example, in the infraredregion for visible light polarizers or mirrors.

Various functional layers or coatings may be added to the films andoptical devices of the present invention to alter or improve theirphysical or chemical properties, particularly along the surface of thefilm or device. Such layers or coatings may include, for example, slipagents, low adhesion backside materials, conductive layers, antistaticcoatings or films, barrier layers, flame retardants, UV stabilizers,abrasion resistant materials, optical coatings, and/or substratesdesigned to improve the mechanical integrity or strength of the film ordevice.

Skin layers or coatings may also be added to impart desired barrierproperties to the resulting film or device. Thus, for example, barrierfilms or coatings may be added as skin layers, or as a component in skinlayers, to alter the transmissive properties of the film or devicetowards liquids, such as water or organic solvents, or gases, such asoxygen or carbon dioxide.

Skin layers or coatings may also be added to impart or improve abrasionresistance in the resulting article. Thus, for example, a skin layercomprising particles of silica embedded in a polymer matrix may be addedto an optical film produced in accordance with the invention to impartabrasion resistance to the film, provided, of course, that such a layerdoes not unduly compromise the optical properties required for theapplication to which the film is directed.

Skin layers or coatings may also be added to impart or improve punctureand/or tear resistance in the resulting article. Factors to beconsidered in selecting a material for a tear resistant layer includepercent elongation to break, Young's modulus, tear strength, adhesion tointerior layers, percent transmittance and absorbance in anelectromagnetic bandwidth of interest, optical clarity or haze,refractive indices as a function of frequency, texture and roughness,melt thermal stability, molecular weight distribution, melt rheology andcoextrudability, miscibility and rate of inter-diffusion betweenmaterials in the skin and optical layers, viscoelastic response,relaxation and crystallization behavior under draw conditions, thermalstability at use temperatures, weatherability, ability to adhere tocoatings and permeability to various gases and solvents. Puncture ortear resistant skin layers may be applied during the manufacturingprocess or later coated onto or laminated to the multilayered polymerfilm 10. Adhering these layers to the film during the manufacturingprocess, such as by a coextrusion process, provides the advantage thatthe film is protected during the manufacturing process. In someembodiments, one or more puncture or tear resistant layers may beprovided within the film, either alone or in combination with a punctureor tear resistant skin layer.

The films and optical devices of the present invention may be given goodslip properties by treating them with low friction coatings or slipagents, such as polymer beads coated onto the surface. Alternately, themorphology of the surfaces of these materials may be modified, asthrough manipulation of extrusion conditions, to impart a slipperysurface to the film; methods by which surface morphology may be somodified are described in U.S. Pat. No. 5,759,467.

In some applications, as where the multilayered polymer films 10 of thepresent invention are to be used as a component in adhesive tapes, itmay be desirable to treat the films with low adhesion backsize (LAB)coatings or films such as those based on urethane, silicone orfluorocarbon chemistry. Films treated in this manner will exhibit properrelease properties towards pressure sensitive adhesives (PSAs), therebyenabling them to be treated with adhesive and wound into rolls. Adhesivetapes made in this manner can be used for decorative purposes or in anyapplication where a diffusely reflective or transmissive surface on thetape is desirable.

The films and optical devices of the present invention may also beprovided with one or more conductive layers. Such conductive layers mayinclude metals such as silver, gold, copper, aluminum, chromium, nickel,tin, and titanium, metal alloys such as silver alloys, stainless steel,and inconel, and semiconductor metal oxides such as doped and undopedtin oxides, zinc oxide, and indium tin oxide (ITO).

The films and optical devices of the present invention may also beprovided with antistatic coatings or films. Such coatings or filmsinclude, for example, V₂O₅ and salts of sulfonic acid polymers, carbonor other conductive metal layers.

The films and devices of the present invention may also be provided withone or more barrier films or coatings that alter the transmissiveproperties of the film towards certain liquids or gases. Thus, forexample, the devices and films of the present invention may be providedwith films or coatings that inhibit the transmission of water vapor,organic solvents, O₂, or CO₂ through the film. Barrier coatings may beparticularly desirable in high humidity environments, where componentsof the film or device may be subject to distortion due to moisturepermeation.

The films and optical devices of the present invention may also betreated with flame retardants, particularly when used in environments,such as on airplanes, that are subject to strict fire codes. Suitableflame retardants include aluminum trihydrate, antimony trioxide,antimony pentoxide, and flame retarding organophosphate compounds.

The films and optical devices of the present invention may also beprovided with abrasion-resistant or hard coatings, which may be appliedas a skin layer. These include acrylic hardcoats such as those availableunder the trade designations Acryloid A-11 and Paraloid K-120N from Rohm& Haas, Philadelphia, Pa.; urethane acrylates, such as those describedin U.S. Pat. No. 4,249,011 and those available from Sartomer Corp.,Westchester, Pa.; and urethane hardcoats obtained from the reaction ofan aliphatic polyisocyanate (e.g., those available under the tradedesignation Desmodur N-3300 from Miles, Inc., Pittsburgh, Pa.) with apolyester (e.g., those available under the trade designation Tone Polyol0305 from Union Carbide, Houston, Tex.).

The films and optical devices of the present invention may further belaminated to rigid or semi-rigid substrates, such as, for example,glass, metal, acrylic, polyester, and other polymer backings to providestructural rigidity, weatherability, or easier handling. For example,the multilayered polymer films 10 may be laminated to a thin acrylic ormetal backing so that it can be stamped or otherwise formed andmaintained in a desired shape. For some applications, such as when thefilm is applied to other breakable backings, an additional layercomprising PET film or puncture-tear resistant film may be used.

The films and optical devices of the present invention may also beprovided with shatter resistant films and coatings. Films and coatingssuitable for this purpose are described, for example, in publications EP592284 and EP 591055, and are available commercially from 3M Company,St. Paul, Minn.

Various optical layers, materials, and devices may also be applied to,or used in conjunction with, the films and devices of the presentinvention for specific applications. These include, but are not limitedto, magnetic or magneto-optic coatings or films; liquid crystal panels,such as those used in display panels and privacy windows; photographicemulsions; fabrics; prismatic films, such as linear Fresnel lenses;brightness enhancement films; holographic films or images; embossablefilms; anti-tamper films or coatings; IR transparent films for lowemissivity applications; release films or release coated paper; andpolarizers or mirrors.

Multiple additional layers on one or both major surfaces of themultilayered polymer film 10 are contemplated, and can be anycombination of the aforementioned coatings or films. For example, whenan adhesive is applied to the multilayered polymer film 10, the adhesivemay contain a white pigment such as titanium dioxide to increase theoverall reflectivity, or it may be optically transparent to allow thereflectivity of the substrate to add to the reflectivity of themultilayered polymer film 10.

In order to improve roll formation and convertibility of the film, themultilayered polymer films 10 of the present invention may also includea slip agent that is incorporated into the film or added as a separatecoating. In most applications, slip agents are added to only one side ofthe film, ideally the side facing the rigid substrate in order tominimize haze.

The films and other optical devices made in accordance with theinvention may also include one or more anti-reflective layers orcoatings, such as, for example, conventional vacuum coated dielectricmetal oxide or metal/metal oxide optical films, silica sol gel coatings,and coated or coextruded anti-reflective layers such as those derivedfrom low index fluoropolymers such as THV, an extrudable fluoropolymeravailable from 3M Company (St. Paul, Minn.). Such layers or coatings,which may or may not be polarization sensitive, serve to increasetransmission and to reduce reflective glare, and may be imparted to thefilms and optical devices of the present invention through appropriatesurface treatment, such as coating or sputter etching.

The films and other optical devices made in accordance with theinvention may be provided with a film or coating which impartsanti-fogging properties. In some cases, an anti-reflection layer asdescribed above will serve the dual purpose of imparting bothanti-reflection and anti-fogging properties to the film or device.Various anti-fogging agents are known to the art. Typically, however,these materials include substances, such as fatty acid esters, whichimpart hydrophobic properties to the film surface and which promote theformation of a continuous, less opaque film of water.

Coatings which reduce the tendency for surfaces to “fog” have beenreported by several inventors. For example, U.S. Pat. No. 3,212,909 toLeigh discloses the use of ammonium soap, such as alkyl ammoniumcarboxylates in admixture with a surface active agent which is asulfated or sulfonated fatty material, to produce a anti-foggingcomposition. U.S. Pat. No. 3,075,228 to Elias discloses the use of saltsof sulfated alkyl aryloxypolyalkoxy alcohol, as well as alkylbenzenesulfonates, to produce an anti-fogging article useful in cleaning andimparting anti-fogging properties to various surfaces. U.S. Pat. No.3,819,522 to Zmoda, discloses the use of surfactant combinationscomprising derivatives of decyne diol as well as surfactant mixtureswhich include ethoxylated alkyl sulfates in an anti-fogging windowcleaner surfactant mixture. Japanese Patent Kokai No. Hei 6[1994]41,335discloses a clouding and drip preventive composition comprisingcolloidal alumina, colloidal silica and an anionic surfactant. U.S. Pat.No. 4,478,909 (Taniguchi et al) discloses a cured anti-fogging coatingfilm which comprises polyvinyl alcohol, a finely divided silica, and anorganic silicon compound, the carbon/silicon weight ratio apparentlybeing important to the film's reported anti-fogging properties. Varioussurfactants, include fluorine-containing surfactants, may be used toimprove the surface smoothness of the coating. Other anti-fog coatingsincorporating surfactants are described in U.S. Pat. Nos. 2,803,552;3,022,178; and 3,897,356. PCT 96/18,691 (Scholtz et al) discloses meansby which coatings may impart both anti-fog and anti-reflectiveproperties.

The films and optical devices of the present invention may be protectedfrom UV radiation through the use of UV stabilized films or coatings.Suitable UV stabilized films and coatings include those whichincorporate benzotriazoles or hindered amine light stabilizers (HALS)such as those available under the trade designation Tinuvin 292 fromCiba Geigy Corp., Hawthorne, N.Y. Other suitable UV stabilized films andcoatings include those which contain benzophenones or diphenylacrylates, available commercially from BASF Corp., Parsippany, N.J. Suchfilms or coatings may be particularly desirable when the films andoptical devices of the present invention are used in outdoorapplications or in luminaires where the source emits significant amountof light in the UV region of the spectrum.

The films and optical devices of the present invention may be treatedwith inks, dyes, or pigments to alter their appearance or to customizethem for specific applications. Thus, for example, the films may betreated with inks or other printed indicia such as those used to displayproduct identification, advertisements, warnings, decoration, or otherinformation. Various techniques may be used to print on the film, suchas screen printing, letterpress, offset, flexographic printing, stippleprinting, laser printing, and so forth, and various types of ink can beused, including one and two component inks, oxidatively drying andUV-drying inks, dissolved inks, dispersed inks, and 100% ink systems. Inaddition, dyes or pigments may be blended into a polymer either beforeor after formation of layers using the polymer.

The appearance of the multilayered polymer film 10 may also be alteredby coloring the film, such as by laminating a dyed film to themultilayered polymer film, applying a pigmented coating to the surfaceof the film, or including a pigment in one or more of the materials usedto make the film.

Both visible and near IR dyes and pigments are contemplated in thepresent invention, and include, for example, optical brighteners such asdyes that absorb in the UV and fluoresce in the visible region of thecolor spectrum. Other additional layers that may be added to alter theappearance of the optical film include, for example, opacifying (black)layers, diffusing layers, holographic images or holographic diffusers,and metal layers. Each of these may be applied directly to one or bothsurfaces of film, or may be a component of a second film or foilconstruction that is laminated to the film. Alternately, some componentssuch as opacifying or diffusing agents, or colored pigments, may beincluded in an adhesive layer which is used to laminate the film toanother surface.

The films and devices of the present invention may also be provided withmetal coatings. Thus, for example, a metallic layer may be applieddirectly to the optical film by pyrolysis, powder coating, vapordeposition, cathode sputtering, ion plating, and the like. Metal foilsor rigid metal plates may also be laminated to the optical film, orseparate polymeric films or glass or plastic sheets may be firstmetallized using the aforementioned techniques and then laminated to thefilms and devices of the present invention.

A brief description of one method for forming multilayer polymer filmsis described. A fuller description of the process conditions andconsiderations is found in U.S. patent application Ser. No. 09/006,288entitled “Process for Making Multilayer Optical Film.” The multilayerpolymer films are formed by extrusion of polymers to be used in thefirst and second optical layers, as well as the non-optical layers.Extrusion conditions are chosen to adequately feed, melt, mix and pumpthe polymer resin feed streams in a continuous and stable manner. Finalmelt stream temperatures are chosen to be within a range which reducesfreezing, crystallization or unduly high pressure drops at the low endof the range and which reduces degradation at the high end of the range.The entire melt stream processing of more than one polymer, up to andincluding film casting on a chill roll, is often referred to asco-extrusion.

Following extrusion, each melt stream is conveyed through a neck tubeinto a gear pump used to regulate the continuous and uniform rate ofpolymer flow. A static mixing unit may be placed at the end of the necktube to carry the polymer melt stream from the gear pump into amultilayer feedblock with uniform melt stream temperature. The entiremelt stream is typically heated as uniformly as possible to enhance bothuniform flow of the melt stream and reduce degradation during meltprocessing.

Multilayer feedblocks divide each of two or more polymer melt streamsinto many layers, interleave these layers, and combine the many layersinto a single multilayer stream. The layers from any given melt streamare created by sequentially bleeding off part of the stream from a mainflow channel into side channel tubes which lead to layer slots in thefeed block manifold. The layer flow is often controlled by choices madein machinery, as well as the shape and physical dimensions of theindividual side channel tubes and layer slots.

The side channel tubes and layer slots of the two or more melt streamsare often interleaved to, for example, form alternating layers. Thefeedblock's downstream-side manifold is often shaped to compress anduniformly spread the layers of the combined multilayer stacktransversely. Thick, non-optical layers, known as protective boundarylayers (PBLs), may be fed near the manifold walls using the melt streamsof the optical multilayer stack, or by a separate melt stream. Asdescribed above, these non-optical layers may be used to protect thethinner optical layers from the effects of wall stress and possibleresulting flow instabilities.

The multilayer stack exiting the feedblock manifold may then enter afinal shaping unit such as a die. Alternatively, the stream may besplit, preferably normal to the layers in the stack, to form two or moremultilayer streams that may be recombined by stacking. The stream mayalso be split at an angle other than normal to the layers. A flowchanneling system that splits and stacks the streams is called amultiplier. The width of the split streams (i.e., the sum of thethicknesses of the individual layers) can be equal or unequal. Themultiplier ratio is defined as the ratio of the wider to narrower streamwidths. Unequal streams widths (i.e., multiplier ratios greater thanunity) can be useful in creating layer thickness gradients. In the caseof unequal stream widths, the multiplier may spread the narrower streamand/or compress the wider stream transversely to the thickness and flowdirections to ensure matching layer widths upon stacking.

Prior to multiplication, additional non-optical layers can be added tothe multilayer stack. These non-optical layers may perform as PBLswithin the multiplier. After multiplication and stacking, some of theselayers may form internal boundary layers between optical layers, whileothers form skin layers.

After multiplication, the web is directed to the final shaping unit. Theweb is then cast onto a chill roll, sometimes also referred to as acasting wheel or casting drum. This casting is often assisted byelectrostatic pinning, the details of which are well-known in the art ofpolymer film manufacture. The web may be cast to a uniform thicknessacross the web or a deliberate profiling of the web thickness may beinduced using die lip controls.

The multilayer web is then drawn to produce the final multilayer opticalfilm. In one exemplary method for making a multilayer optical polarizer,a single drawing step is used. This process may be performed in a tenteror a length orienter. Typical tenters draw transversely (TD) to the webpath, although certain tenters are equipped with mechanisms to draw orrelax (shrink) the film dimensionally in the web path or machinedirection (MD). Thus, in this exemplary method, a film is drawn in onein-plane direction. The second in-plane dimension is either heldconstant as in a conventional tenter, or is allowed to neck in to asmaller width as in a length orienter. Such necking in may besubstantial and increase with draw ratio.

In one exemplary method for making a multilayer mirror, a two stepdrawing process is used to orient the birefringent material in bothin-plane directions. The draw processes may be any combination of thesingle step processes described that allow drawing in two in-planedirections. In addition, a tenter that allows drawing along MD, e.g. abiaxial tenter which can draw in two directions sequentially orsimultaneously, may be used. In this latter case, a single biaxial drawprocess may be used.

In still another method for making a multilayer polarizer, a multipledrawing process is used that exploits the different behavior of thevarious materials to the individual drawing steps to make the differentlayers comprising the different materials within a single coextrudedmultilayer film possess different degrees and types of orientationrelative to each other. Mirrors can also be formed in this manner.

The intrinsic viscosity of the polyesters used in these layers and filmsis related to the molecular weight (in the absence of branchingmonomers) of the polymer. Typically, the polyesters have an intrinsicviscosity of greater than about 0.4 dL/g. Preferably, the intrinsicviscosity is between about 0.4 to 0.7 dL/g. Intrinsic viscosity, forpurposes of this disclosure, is measured in a 60/40 wt. %phenol/o-dichlorobenzene solvent at 30° C. unless otherwise indicated.

The following examples demonstrate the manufacture and uses ofmultilayered polymer films of the invention. It is to be understood thatthese examples are merely illustrative and are in no way to beinterpreted as limiting the scope of the invention.

EXAMPLES

Monomers, catalysts, and stabilizers utilized in creating polymers forthese examples are commercially available from the following suppliers:dimethyl naphthalene dicarboxylate and terephthalic acid from Amoco(Decatur, Ala.), dimethyl terephthalate from Hoechst Celanese (Dallas,Tex.), dimethyl isophthalate and dimethyl tertiary-butyl isophthalatefrom Morflex Inc. (Greensboro, N.C.), ethylene glycol from Union Carbide(Charleston, W. Va.), 1,6-hexanediol from BASF (Charlotte, N.C.),sebacic acid from Union Camp (Dover, Ohio), antimony triacetate from ElfAtochem (Philadelphia, Pa.), cobalt acetate and manganese acetate fromHall Chemical (Wickliffe, Ohio), triethyl phosphonoacetate from Albright& Wilson (Glen Allen, Va.), dimethyl cyclohexane dicarboxylate fromEastman Chemical Co. (Kingsport, Tenn.), and triethylamine from AirProducts (Phillipsburg, N.J.).

In each of the examples described below, an 836 layer film is formed.The 836 optical layer construction includes four multilayer opticalstacks of graded layer thicknesses as obtained by the doublemultiplication of a 209 layer construction from a multilayer feed block.The optical layers account for approximately 50 percent of the thicknessof the construction. Each of the stacks is separated by one of threenon-optical internal protective boundary layers, each accounting forabout 2% of the total thickness. Finally, each side of the filmpossesses an outer non-optical skin layer, each accounting forapproximately 22% of the thickness.

A “gain tester” was used to test several of the films in the Examples.The “gain tester” can be fabricated using a spot photometer and asuitable backlight with a polarizer placed between the two so that onlyone polarization of light from the backlight is measured by thephotometer. Suitable spot photometers include the Minolta LS-100 andLS-110 (Ramsey, N.J.). The absolute value of a measured gain on thebacklight used and on the orientation of the sample on the backlight, aswell as the size of the sample. The backlight used in the Examples wasobtained from Landmark and the polarizer was a high contrast displaypolarizer which was oriented so that the pass axis of the polarizer wasaligned with the long axis of the backlight. The sample was insertedinto the tester so that the pass axis of the sample was aligned with thepass axis of the high contrast polarizer. The sample was made largeenough to cover the entire backlight.

Comparative Example

Polarizing film with PEN/coPEN (70/0/30) layers. As a comparativeexample, a multilayer reflective polarizer film was constructed withfirst optical layers created from polyethylene naphthalate and secondoptical layers created from co(polyethylene naphthalate) withcarboxylate subunits derived from 70 mol % dimethyl naphthalenedicarboxylate and 30 mol % dimethyl isophthalate, and glycol subunitsderived from 100 mol % ethylene glycol.

The polyethylene naphthalate used to form the first optical layers wassynthesized in a batch reactor with the following raw material charge:136 kg dimethyl naphthalene dicarboxylate, 73 kg ethylene glycol, 27 gmanganese acetate, 27 g cobalt acetate, and 48 g antimony triacetate.Under pressure of 2 atm (2×10⁵ N/m²), this mixture was heated to 254° C.while removing methanol (a transesterification reaction by-product).After 35 kg of methanol was removed, 49 g of triethyl phosphonoacetatewas charged to the reactor and than the pressure was gradually reducedto 1 torr while heating to 290° C. The condensation reaction by-product,ethylene glycol, was continuously removed until a polymer with anintrinsic viscosity of 0.48 dL/g, as measured in 60/40 wt. %phenol/o-dichlorobenzene, was produced.

The co(polyethylene naphthalate) used to form the second optical layerswas synthesized in a batch reactor with the following raw materialcharge: 109 kg dimethyl naphthalene dicarboxylate, 37 kg dimethylisophthalate, 79 kg ethylene glycol, 29 g manganese acetate, 29 g cobaltacetate, and 58 g antimony triacetate. Under pressure of 2 atm (2×10⁵N/m²), this mixture was heated to 254° C. while removing methanol. After41 kg of methanol was removed, 52 g of triethyl phosphonoacetate wascharged to the reactor and than the pressure was gradually reduced to 1torr while heating to 290° C. The condensation reaction by-product,ethylene glycol, was continuously stripped until a polymer with anintrinsic viscosity of 0.57 dL/g, as measured in 60/40 wt. %phenol/o-dichlorobenzene, was produced.

The above described PEN and coPEN were then coextruded throughmultilayer melt manifolds to create a multilayer film with 836alternating first and second optical layers. This multilayer reflectivefilm also contains internal protective layers and external protectivelayers made of the same co(polyethylene naphthalate) as the secondoptical layers. These protective layers are introduced throughadditional melt ports. This cast film was heated in an oven charged withhot air set at 150° C. for about one minute and then uniaxially orientedat a 6:1 draw to produce a reflective polarizer film of approximately125 μm thickness.

When the described multilayer reflective film was placed within a “gaintester”, as described above, the brightness increased by 58% whichcorrelates to a “gain” of 1.58. Increases in brightness are measured asgain, which is the ratio of the brightness of a tester with thepolarizing film to the brightness of the tester without the polarizingfilm.

A second film was constructed and processed as above, except that thissecond film was uniaxially oriented at a 7:1 draw. The resultingbirefringence of the second film was estimated to be about 0.24 at 632.8nm. The average gain of the second film was estimated to be about 1.62.

A peel strength test was performed. Samples of the second film were cutin 2.54 cm strips at 45° with respect to the reflection and transmissionaxes (i.e., in-plane axes) of the film. The multilayer optical film wasadhered to a substrate and then, using an Instrumentors, Inc. slip/peeltester (Strongsville, Ohio), the layers of the film were peeled away at2.54 cm/second at 25° C., 50% relative humidity, and 90° peel angle. Theerror in the test was estimated to be about ±8×10³ dynes/cm. For thissecond film, the resistance to delamination between the two set ofoptical layers was about 1.2×10⁴ dynes/cm, which is relatively low.

A third multilayer reflective polarizer film was constructed andprocessed in a manner similar to the first two films except that thefilm was preheated in a tenter with hot air charged at a temperature ofabout 160° C. and then drawn with the air charged at about 150° C. Thein-plane birefringence of this film was estimated to be about 0.17 for632.8 nm light. The average gain was estimated to be about 1.53. Theresistance to delamination was about 6.2×10⁴ dynes/cm.

Example 1

Polarizing film with coPEN (90/10/0)/coPEN (55/0/45) layers. Amultilayer reflective polarizer film may be constructed with firstoptical layers created from a co(polyethylene naphthalate) withcarboxylate subunits derived from 90 mol % dimethyl naphthalenedicarboxylate and 10 mol % dimethyl terephthalate, and glycol subunitsderived from 100 mol % ethylene glycol subunits, and second opticallayers created from a co(polyethylene naphthalate) with carboxylatesubunits derived from 55 mol % dimethyl naphthalene dicarboxylate and 45mol % dimethyl isophthalate, and glycol subunits derived from 99.8 mol %ethylene glycol and 0.2 mol % trimethylol propane.

The co(polyethylene naphthalate) used to form the first optical layersis synthesized in a batch reactor with the following raw materialcharge: 126 kg dimethyl naphthalene dicarboxylate, 11 kg dimethylterephthalate, 75 kg ethylene glycol, 27 g manganese acetate, 27 gcobalt acetate, and 48 g antimony triacetate. Under pressure of 2 atm(2×10⁵ N/m²), this mixture is heated to 254° C. while removing methanol.After 36 kg of methanol is removed, 49 g of triethyl phosphonoacetate ischarged to the reactor and than the pressure is gradually reduced to 1torr while heating to 290° C. The condensation reaction by-product,ethylene glycol, is continuously removed until a polymer with anintrinsic viscosity of 0.50 dL/g, as measured in 60/40 wt. %phenol/o-dichlorobenzene, is produced.

The co(polyethylene naphthalate) used to form the second optical layersis synthesized in a batch reactor with the following raw materialcharge: 83 kg dimethyl naphthalene dicarboxylate, 54 kg dimethylisophthalate, 79 kg ethylene glycol, 313 g trimethylol propane, 27 gramsmanganese acetate, 27 grams cobalt acetate, and 48 g antimonytriacetate. Under pressure of 2 atm (2×10⁵ N/m²), this mixture is heatedto 254° C. while removing methanol. After 39.6 kg of methanol isremoved, 49 g of triethyl phosphonoacetate is charged to the reactor andthan the pressure is gradually reduced to 1 torr while heating to 290°C. The condensation reaction by-product, ethylene glycol, iscontinuously stripped until a polymer with an intrinsic viscosity of0.60 dL/g, as measured in 60/40 wt. % phenol/o-dichlorobenzene, isproduced.

The above described coPEN's are then coextruded through a multilayermelt manifold to create a multilayer film with 836 alternating first andsecond optical layers. This particular multilayer reflective film alsocontains internal and external protective layers made of the sameco(polyethylene naphthalate) as the second optical layers. The cast filmis heated in an oven charged with hot air set at 145° C. for about oneminute and then uniaxially oriented at a 6:1 draw to produce areflective polarizer of approximately 125 μm thickness.

Example 2

Polarizing film with coPEN (85/15/0)/coPEN (50/0/50) layers. Amultilayer reflective polarizer film was constructed with first opticallayers created from a co(polyethylene naphthalate) with carboxylatesubunits derived from 85 mol % dimethyl naphthalene dicarboxylate and 15mol % dimethyl terephthalate, and glycol subunits derived from 100 mol %ethylene glycol, and second optical layers created from aco(polyethylene naphthalate) with carboxylate subunits derived from 50mol % dimethyl naphthalene dicarboxylate and 50 mol % dimethylisophthalate, and glycol subunits derived from 100 mol % ethyleneglycol.

The co(polyethylene naphthalate) used to form the first optical layerswas synthesized in a batch reactor with the following raw materialcharge: 123 kg dimethyl naphthalene dicarboxylate, 17 kg dimethylterephthalate, 76 kg ethylene glycol, 27 g manganese acetate, 27 gcobalt acetate, and 48 g antimony triacetate. Under pressure of 2 atm(2×10⁵ N/m²), this mixture was heated to 254° C. while removingmethanol. After 36 kg of methanol was removed, 49 g of triethylphosphonoacetate was charged to the reactor and than the pressure wasgradually reduced to 1 torr while heating to 290° C. The condensationreaction by-product, ethylene glycol, was continuously removed until apolymer with an intrinsic viscosity of 0.51 dL/g, as measured in 60/40wt. % phenol/o-dichlorobenzene, was produced.

The co(polyethylene naphthalate) used to form the second optical layerswas synthesized in a batch reactor with the following raw materialcharge: 77 kg dimethyl naphthalene dicarboxylate, 61 kg dimethylisophthalate, 82 kg ethylene glycol, 27 grams manganese acetate, 27grams cobalt acetate, and 48 g antimony triacetate. Under pressure of 2atm (2×10⁵ N/m²), this mixture was heated to 254° C. while removingmethanol. After 39.6 kg of methanol was removed, 49 g of triethylphosphonoacetate was charged to the reactor and than the pressure wasgradually reduced to 1 torr while heating to 290° C. The condensationreaction by-product, ethylene glycol, was continuously stripped until apolymer with an intrinsic viscosity of 0.60 dL/g, as measured in 60/40wt. % phenol/o-dichlorobenzene, was produced.

The above described coPEN's were then coextruded through a multilayermelt manifold to create a multilayer film with 836 alternating first andsecond optical layers. This particular multilayer reflective film alsocontained internal and external protective layers made of the sameco(polyethylene naphthalate) as the second optical layers. The cast filmwas heated in an oven charged with hot air set at 135° C. for about oneminute and then uniaxially oriented at a 6:1 draw to produce areflective polarizer of approximately 125 μm thickness. The resultingin-plane birefringence was estimated to be about 0.17 for 632.8 nmlight. The resistance to interlayer delamination was about 5.9×10⁴dynes/cm.

When the described multilayer reflective film was placed within a “gaintester”, as described above, the brightness increased by 58% whichcorrelates to a “gain” of 1.58. Increases in brightness are measured asgain, which is the ratio of the brightness of a tester with thepolarizing film to the brightness of the tester without the polarizingfilm.

A second film was formed in the same manner except that the second filmwas drawn in hot air charged to 129° C. The resulting in-planebirefringence was estimated to be about 0.185. The measured gain was1.58 and the resistance to interlayer delamination was about 4.5×10⁴dynes/cm.

Example 3

Polarizing film with coPEN (88/12/0)/coPEN (55/45/0) layers. Amultilayer reflective polarizer film was constructed with first opticallayers created from a co(polyethylene naphthalate) with carboxylatesubunits derived from 88 mol % dimethyl naphthalene dicarboxylate and 12mol % dimethyl terephthalate, and glycol subunits derived from 100 mol %ethylene glycol, and second optical layers created from aco(polyethylene naphthalate) with carboxylate subunits derived from 55mol % dimethyl naphthalene dicarboxylate and 45 mol % dimethylterephthalate, and glycol subunits derived from 96.8 mol % ethyleneglycol, 3.0 mol % hexanediol, and 0.2 mol % trimethylol propane.

The co(polyethylene naphthalate) used to form the first optical layerswas created as a blend of two polymers: a PET (8 wt. %) and a coPEN (92wt. %). The PET used in the blend was synthesized in a batch reactorwith the following raw material charge: 138 kg dimethyl terephthalate,93 kg ethylene glycol, 27 g zinc acetate, 27 g cobalt acetate, and 48 gantimony triacetate. Under pressure of 2 atm (2×10⁵ N/m²), this mixturewas heated to 254° C. while removing the transesterification reactionby-product, methanol. After 45 kg of methanol was removed 52 g oftriethyl phosphonoacetate was charged to the reactor and then thepressure was gradually reduced to 1 torr while heating to 290° C. Thecondensation reaction by-product, ethylene glycol, was continuouslyremoved until a polymer with an intrinsic viscosity of 0.60, as measuredin 60/40 wt. % phenol/o-dichlorobenzene, was produced.

The coPEN used in the blend to form the first optical layers hadcarboxylate subunits that were derived from 97 mol % dimethylnaphthalene dicarboxylate and 3 mol % dimethyl terephthalate and glycolsubunits derived from 100 mol % ethylene glycol. The coPEN wassynthesized in a batch reactor with the following raw material charge:135 kg dimethyl naphthalene dicarboxylate, 3.2 kg dimethylterephthalate, 75 kg ethylene glycol, 27 g manganese acetate, 27 gcobalt acetate, and 48 g antimony triacetate. Under pressure of 2 atm(2×10⁵ N/m²), this mixture was heated to 254° C. while removingmethanol. After 37 kg of methanol was removed, 49 g of triethylphosphonoacetate was charged to the reactor and than the pressure wasgradually reduced to 1 torr while heating to 290° C. The condensationreaction by-product, ethylene glycol, was continuously removed until apolymer with an intrinsic viscosity of 0.50 dL/g, as measured in 60/40wt. % phenol/o-dichlorobenzene, was produced.

The co(polyethylene naphthalate) used to form the second optical layerswas synthesized in a batch reactor with the following raw materialcharge: 88.5 kg dimethyl naphthalene dicarboxylate, 57.5 kg dimethylterephthalate, 81 kg ethylene glycol, 4.7 kg hexane diol, 15 gramsmanganese acetate, 22 grams cobalt acetate, 15 g zinc acetate, 239 gtrimethylol propane, and 51 g antimony triacetate. Under pressure of 2atm (2×10⁵ N/m²), this mixture was heated to 254° C. while removingmethanol. After 39.6 kg of methanol was removed, 47 g of triethylphosphonoacetate was charged to the reactor and than the pressure wasgradually reduced to 1 torr while heating to 290° C. The condensationreaction by-product, ethylene glycol, was continuously stripped until apolymer with an intrinsic viscosity of 0.56 dL/g, as measured in 60/40wt. % phenol/o-dichlorobenzene, was produced.

The above described coPEN's were then coextruded through a multilayermelt manifold to create a multilayer film with 836 alternating first andsecond optical layers. This particular multilayer reflective film alsocontained internal and external protective layers made of the sameco(polyethylene naphthalate) as the second optical layers. The cast filmwas heated in an oven charged with hot air set at 140° C. for about oneminute and then uniaxially oriented at a 6:1 draw to produce areflective polarizer of approximately 125 μm thickness.

When the described multilayer reflective film was placed within a “gaintester”, as described above, the brightness increased by 58% whichcorrelates to a “gain” of 1.58. Increases in brightness are measured asgain, which is the ratio of the brightness of a tester with thepolarizing film to the brightness of the tester without the polarizingfilm.

Interlayer adhesion was measured to be about 9.5×10⁴ dynes/cm using the90 degree tape peel test.

Example 4

Polarizing film with coPEN (85/15/0)/coPEN (55/45/0) layers. Amultilayer reflective polarizer film was constructed with first opticallayers created from a co(polyethylene naphthalate) with carboxylatesubunits derived from 85 mol % dimethyl naphthalene dicarboxylate and 15mol % dimethyl terephthalate, and glycol subunits derived from 100 mol %ethylene glycol subunits, and second optical layers created from aco(polyethylene naphthalate) with carboxylate subunits derived from 55mol % dimethyl naphthalene dicarboxylate and 45 mol % dimethylterephthalate and glycol subunits derived from 96.8 mol % ethyleneglycol, 3.0 mol % hexane diol, and 0.2 mol % trimethylol propane.

The co(polyethylene naphthalate) used to form the first optical layerswas synthesized as in Example 2.

The co(polyethylene naphthalate) used to form the second optical layerswas synthesized as in Example 3.

The above described coPEN's were then coextruded through a multilayermelt manifold to create a multilayer film with 836 alternating first andsecond optical layers. This particular multilayer reflective film alsocontained internal and external protective layers made of the sameco(polyethylene naphthalate) as the second optical layers. The cast filmwas heated in an oven charged with hot air set at 135° C. for about oneminute and then uniaxially oriented at a 6:1 draw to produce areflective polarizer of approximately 125 μm thickness.

When the described multilayer reflective film was placed within a “gaintester”, as described above, the brightness increased by 58% whichcorrelates to a “gain” of 1.58. Increases in brightness are measured asgain, which is the ratio of the brightness of a tester with thepolarizing film to the brightness of the tester without the polarizingfilm.

Example 5

Polarizing film with coPEN (85/15/0)/coPEN (50/50/0) layers. Amultilayer reflective polarizer film was constructed with first opticallayers created from a co(polyethylene naphthalate) with carboxylatesubunits derived from 85 mol % dimethyl naphthalene dicarboxylate and 15mol % dimethyl terephthalate and glycol subunits derived from 100 mol %ethylene glycol, and second optical layers created from aco(polyethylene naphthalate) with carboxylate subunits derived from 50mol % dimethyl naphthalene dicarboxylate and 50 mol % dimethylterephthalate and glycol subunits derived from 96.8 mol % ethyleneglycol, 3.0 mol % hexane diol, and 0.2 mol % trimethylol propane.

The co(polyethylene naphthalate) used to form the first optical layerswas synthesized as in Example 2.

The co(polyethylene naphthalate) used to form the second optical layerswas synthesized in a batch reactor with the following raw materialcharge: 81.4 kg dimethyl naphthalene dicarboxylate, 64.5 kg dimethylterephthalate, 82 kg ethylene glycol, 4.7 kg hexane diol, 15 g manganeseacetate, 22 g cobalt acetate, 15 g zinc acetate, 239 g trimethylolpropane, and 48 g antimony triacetate. Under pressure of 2 atm (2×10⁵N/m²), this mixture was heated to 254° C. while removing methanol. After44 kg of methanol was removed, 47 g of triethyl phosphonoacetate wascharged to the reactor and than the pressure was gradually reduced to 1torr while heating to 290° C. The condensation reaction by-product,ethylene glycol, was continuously stripped until a polymer with anintrinsic viscosity of 0.60 dL/g, as measured in 60/40 wt. %phenol-dichlorobenzene, was produced.

The above described coPEN's were then coextruded through a multilayermelt manifold to create a multilayer film with 836 alternating first andsecond optical layers. This particular multilayer reflective film alsocontained internal and external protective layers made of the sameco(polyethylene naphthalate) as the second optical layers. The cast filmwas heated in an oven charged with hot air set at 135° C. for about oneminute and then uniaxially oriented at a 6:1 draw to produce areflective polarizer of approximately 125 μm thickness.

When the described multilayer reflective film was placed within a “gaintester”, as described above, the brightness increased by 58% whichcorrelates to a “gain” of 1.58. Increases in brightness are measured asgain, which is the ratio of the brightness of a tester with thepolarizing film to the brightness of the tester without the polarizingfilm.

Example 6

Polarizing film with second optical layers derived from dimethylcyclohexane dicarboxylate. A multilayer reflective polarizer film may beconstructed with first optical layers created from a copolyester havingcarboxylate subunits derived from 100 mol % dimethyl terephthalate andglycol subunits derived from 90 mol % 1,4-butanediol and 10 mol %ethylene glycol. The second optical layers are made from a copolyesterwhich has carboxylate subunits derived from 50 mol % cyclohexanedicarboxylic acid and 50 mol % terephthalic acid and glycol subunitsderived from 99.8 mol % ethylene glycol, and 0.2 mol % trimethylolpropane.

The poly(butylene terephthalate) used to form the first optical layersis synthesized in a batch reactor with the following raw materialcharge: 127 kg dimethyl terephthalate, 77 kg 1,4-butanediol, 9 kgethylene glycol, and 11 g tetrabutyl titanate. Under pressure of 2 atm(2×10⁵ N/m²), this mixture is heated to 254° C. while removing thetransesterification reaction by-product, methanol. After removing 41 kgof methanol, the reactor pressure is reduced to atmospheric pressure andexcess 1,4-butanediol is removed. Another 22 grams of tetrabutyltitanate is then charged to the reactor and the pressure is furtherreduced to 1 torr while heating to 270° C. The polycondensationby-product, 1,4-butanediol, is continuously stripped until a polymerwith an intrinsic viscosity of 0.85 dL/g, as measured in 60/40 wt. %phenol/o-dichlorobenzene, is produced.

The copolyester used to form the second optical layers is synthesized ina batch reactor with the following raw material charge: 58.6terephthalic acid 59.5 kg cyclohexane dicarboxylic acid, 87.7 kgethylene glycol, 300 g triethyl amine, 275 g trimethylol propane, and 82g antimony triacetate. Under pressure of 2 atm (2×10⁵ N/m²), thismixture is heated to 254° C. while removing the transesterificationreaction by-product, water. After 25.5 kg of water is removed, thepressure is gradually reduced to 1 torr while heating to 290° C. Thecondensation reaction by-product, ethylene glycol, is continuouslystripped until a polymer with an intrinsic viscosity of 1.1 dL/g, asmeasured in 60/40 wt. % phenol/o-dichlorobenzene, is produced.

The above-described copolyesters are then coextruded through amultilayer melt manifold to create a multilayer film with 836alternating first and second optical layers. This particular multilayerreflective film also contains internal protective layers and externalprotective layers made from of the same copolyester as the secondoptical layers. The cast film is heated in an oven charged with hot airset at 65° C. for about one minute and then uniaxially oriented at a 6:1draw to produce a reflective polarizer of approximately 125 μmthickness.

Example 7

Mirror film with second optical layers derived from dimethyl cyclohexanedicarboxylate and tertiary isophthalate. A multilayer reflective mirrorfilm may be constructed with first optical layers created from coPENhaving carboxylate subunits derived from 90 mol % dimethyl naphthalenedicarboxylate and 10 mol % dimethyl terephthalate and glycol subunitsderived 100 mol % ethylene glycol. The second optical layers are madefrom a copolyester which has carboxylate subunits derived from 85 mol %cyclohexane dicarboxylic acid and 15 mol % dimethyl tertiary-butylisophthalate and glycol subunits derived from 99.7 mol % ethyleneglycol, and 0.3 mol % trimethylol propane.

The coPEN used to form the first optical layers is synthesized as inExample 1.

The copolyester used to form the second optical layers is synthesized ina batch reactor with the following raw material charge: 25.5 kg dimethyltertiary-butyl isophthalate, 112 kg cyclohexane dicarboxylic acid, 88 kgethylene glycol, 409 g trimethylol propane, 34 g copper acetate, 27 gmanganese acetate, and 82 g antimony triacetate. Under pressure of 2 atm(2×10⁵ N/m²), this mixture is heated to 254° C. while removing thetransesterification reaction by-product, methanol. After 43 kg ofmethanol is removed, the pressure is gradually reduced to 1 torr whileheating to 290° C. The condensation reaction by-product, ethyleneglycol, is continuously stripped until a polymer with an intrinsicviscosity of 1.2 dL/g, as measured in 60/40 wt. %phenol/o-dichlorobenzene, is produced.

The above-described copolyesters are then coextruded through amultilayer melt manifold to create a multilayer film with 836alternating first and second optical layers. This particular multilayerreflective film also contains internal protective layers and externalprotective layers made from of the same copolyester as the secondoptical layers. This cast film is biaxially oriented. First, the film isheated in an oven charged with hot air set at 120° C. for about oneminute and then oriented at a 3.6:1 draw. Then the film is heated in anoven charged with hot air set at 135° C. for about one minute and thenoriented in a transverse direction at a 4.0:1 draw.

Example 8

Polarizing film with PEN optical layers, low intrinsic viscosity coPEN(70/0/30) optical layers, and higher intrinsic viscosity coPEN (70/0/30)non-optical layers. A multilayer reflective polarizer film may beconstructed with first optical layers created from polyethylenenaphthalate and second optical layers created from a low intrinsicviscosity (0.48 dL/g) co(polyethylene naphthalate) with carboxylatesubunits derived from 70 mol % dimethyl naphthalene dicarboxylate and 30mol % dimethyl isophthalate, and glycol subunits derived from 100 mol %ethylene glycol. The film also includes non-optical layers mad from ahigher intrinsic viscosity (0.57 dL/g) co(polyethylene naphthalate) withcarboxylate subunits derived from 70 mol % dimethyl naphthalenedicarboxylate and 30 mol % dimethyl isophthalate, and glycol subunitsderived from 100 mol % ethylene glycol.

The polyethylene naphthalate used to form the first optical layers issynthesized in a batch reactor with the following raw material charge:136 kg dimethyl naphthalene dicarboxylate, 73 kg ethylene glycol, 27 gmanganese acetate, 27 g cobalt acetate, and 48 g antimony triacetate.Under pressure of 2 atm (2×10⁵ N/m²), this mixture is heated to 254° C.while removing methanol (a transesterification reaction by-product).After 35 kg of methanol is removed, 49 g of triethyl phosphonoacetate ischarged to the reactor and than the pressure is gradually reduced to 1torr while heating to 290° C. The condensation reaction by-product,ethylene glycol, is continuously removed until a polymer with anintrinsic viscosity of 0.46 dL/g, as measured in 60/40 wt. %phenol/o-dichlorobenzene, is produced.

The co(polyethylene naphthalate) used to form the second optical layersis synthesized in a batch reactor with the following raw materialcharge: 109 kg dimethyl naphthalene dicarboxylate, 37 kg dimethylisophthalate, 79 kg ethylene glycol, 29 g manganese acetate, 29 g cobaltacetate, and 58 g antimony triacetate. Under pressure of 2 atm (2×10⁵N/m²), this mixture is heated to 254° C. while removing methanol. After41 kg of methanol is removed, 52 g of triethyl phosphonoacetate ischarged to the reactor and than the pressure is gradually reduced to 1torr while heating to 290° C. The condensation reaction by-product,ethylene glycol, is continuously stripped until a polymer with anintrinsic viscosity of 0.48 dL/g, as measured in 60/40 wt. %phenol/o-dichlorobenzene, is produced.

The co(polyethylene naphthalate) used to form the non-optical layers issynthesized in a batch reactor with the following raw material charge:109 kg dimethyl naphthalene dicarboxylate, 37 kg dimethyl isophthalate,79 kg ethylene glycol, 29 g manganese acetate, 29 g cobalt acetate, and58 g antimony triacetate. Under pressure of 2 atm (2×10⁵ N/m²), thismixture is heated to 254° C. while removing methanol. After 41 kg ofmethanol is removed, 52 g of triethyl phosphonoacetate is charged to thereactor and than the pressure is gradually reduced to 1 torr whileheating to 290° C. The condensation reaction by-product, ethyleneglycol, is continuously stripped until a polymer with an intrinsicviscosity of 0.57 dL/g, as measured in 60/40 wt. %phenol/o-dichlorobenzene, is produced.

The above described PEN and coPEN for the second optical layers may thenbe coextruded through multilayer melt manifolds to create a multilayerfilm with alternating first and second optical layers. This multilayerreflective film also contains internal protective layers and externalprotective layers made using the higher intrinsic viscosityco(polyethylene naphthalate) which are introduced through additionalmelt ports. This cast film is heated in an oven charged with hot air setat 145° C. for about one minute and the uniaxially oriented at a 6:1draw to produce a reflective polarizer film of approximately 125 μmthickness.

Example 9

Polarizing film with coPEN (85/15) optical layers, low intrinsicviscosity coPEN (50/50) optical layers, and higher intrinsic viscositycoPEN (50/50) non-optical layers. A multilayer reflective polarizer filmmay be constructed with first optical layers created fromco(polyethylene naphthalate) with carboxylate subunits derived from 85mol % dimethyl naphthalene dicarboxylate and 15 mol % dimethylterephthalate, and glycol subunits derived from 100 mol % ethyleneglycol and second optical layers created from a low intrinsic viscosity(0.48 dL/g) co(polyethylene naphthalate) with carboxylate subunitsderived from 50 mol % dimethyl naphthalene dicarboxylate and 50 mol %dimethyl terephthalate, and glycol subunits derived from 96.6 mol %ethylene glycol, 3 mol % 1,6-hexanediol, and 0.4 mol % trimethylolpropane. The film also includes non-optical layers mad from a higherintrinsic viscosity (0.56 dL/g) co(polyethylene naphthalate) withcarboxylate subunits derived from 50 mol % dimethyl naphthalenedicarboxylate and 50 mol % dimethyl terephthalate, and glycol subunitsderived from 96.8 mol % ethylene glycol, 3 mol % 1,6-hexanediol, and 0.2mol % trimethylol propane.

The co(polyethylene naphthalate) used to form the first optical layersis synthesized in a batch reactor with the following raw materialcharge: 123 kg dimethyl naphthalene dicarboxylate, 17 kg dimethylterephthalate, 76 kg ethylene glycol, 27 g manganese acetate, 27 gcobalt acetate, and 48 g antimony triacetate. Under pressure of 2 atm(2×10⁵ N/m²), this mixture is heated to 254° C. while removing methanol(a transesterification reaction by-product). After 36 kg of methanol isremoved, 49 g of triethyl phosphonoacetate is charged to the reactor andthan the pressure is gradually reduced to 1 torr while heating to 290°C. The condensation reaction by-product, ethylene glycol, iscontinuously removed until a polymer with an intrinsic viscosity of 0.48dL/g, as measured in 60/40 wt. % phenol/o-dichlorobenzene, is produced.

The co(polyethylene naphthalate) used to form the second optical layersis synthesized in a batch reactor with the following raw materialcharge: 81.4 kg dimethyl naphthalene dicarboxylate, 64.5 kg dimethylterephthalate, 82 kg ethylene glycol, 4.7 kg 1,6-hexanediol, 15 gmanganese acetate, 22 g cobalt acetate, 15 g zinc acetate, 581 gtrimethylol propane, and 48 g antimony triacetate. Under pressure of 2atm (2×10⁵ N/m²), this mixture is heated to 254° C. while removingmethanol. After 44 kg of methanol is removed, 47 g of triethylphosphonoacetate is charged to the reactor and than the pressure isgradually reduced to 1 torr while heating to 290° C. The condensationreaction by-product, ethylene glycol, is continuously stripped until apolymer with an intrinsic viscosity of 0.48 dL/g, as measured in 60/40wt. % phenol/o-dichlorobenzene, is produced.

The co(polyethylene naphthalate) used to form the non-optical layers issynthesized in a batch reactor with the following raw material charge:81.4 kg dimethyl naphthalene dicarboxylate, 64.5 kg dimethylterephthalate, 82 kg ethylene glycol, 4.7 kg 1,6-hexanediol, 15 gmanganese acetate, 22 g cobalt acetate, 15 g zinc acetate, 290 gtrimethylol propane, and 48 g antimony triacetate. Under pressure of 2atm (2×10⁵ N/m²), this mixture is heated to 254° C. while removingmethanol. After 44 kg of methanol is removed, 47 g of triethylphosphonoacetate is charged to the reactor and than the pressure isgradually reduced to 1 torr while heating to 290° C. The condensationreaction by-product, ethylene glycol, is continuously stripped until apolymer with an intrinsic viscosity of 0.56 dL/g, as measured in 60/40wt. % phenol/o-dichlorobenzene, is produced.

The above described coPENs for the first and second optical layers maythen be coextruded through multilayer melt manifolds to create amultilayer film with alternating first and second optical layers. Thismultilayer reflective film also contains internal protective layers andexternal protective layers made using the higher intrinsic viscosityco(polyethylene naphthalate) which are introduced through additionalmelt ports. This cast film is heated in an oven charged with hot air setat 130° C. for about one minute and the uniaxially oriented at a 6:1draw to produce a reflective polarizer film of approximately 125 μmthickness.

Example 10

A multi-layer reflective polarizer film was constructed with firstoptical layers created from polyethylenenaphthalate comprised of 100 mol% naphthalene dicarboxylate as the carboxylate, and 100 mol % ethyleneglycol as the diol. Second optical layers were created fromcopolyethylenenaphthalate comprised of 55 mol % naphthalenedicarboxylate and 45 mol % terephthalate as carboxylates, and 95.8 mol %ethylene glycol, 4 mol % hexane diol, and 0.2 mol % trimethylol propaneas glycols.

Polyethylenenaphthalate used to form the first optical layers wassynthesized in a batch reactor with the following raw material charge;136 kg dimethyl naphthalene dicarboxylate, 73 kg ethylene glycol, 27grams manganese acetate, 27 grams cobalt acetate, and 48 g antimonytri-acetate. Under pressure of 2 atm, this mixture was heated to 254° C.while removing the transesterification reaction by-product methanol.After 35 kg of methanol was removed, 49 g of triethyl phosphonoacetatewas charged to the reactor and than the pressure was gradually reducedto 1 torr while heating to 290° C. The condensation reaction by-product,ethylene glycol, was continuously removed until a polymer with anIntrinsic Viscosity of 0.48, as measured in 60/40phenol/dichlorobenzene, was produced.

Copolyethylenenaphthalate used to form the second optical layers wassynthesized in a batch reactor with the following raw material charge;88.5 kg dimethyl naphthalene dicarboxylate, 57.5 kg dimethylterephthalate, 81 kg ethylene glycol, 4.7 kg hexane diol, 29 gramscobalt acetate, 29 g zinc acetate, 239 g trimethylol propane, and 51 gantimony tri-acetate. Under pressure of 2 atm, this mixture was heatedto 254° C. while removing the transesterification reaction by-productmethanol. After 39.6 kg of methanol was removed, 56 g of triethylphosphonoacetate was charged to the reactor and than the pressure wasgradually reduced to 1 torr while heating to 290° C. The condensationreaction byproduct, ethylene glycol, was continuously stripped until apolymer with an Intrinsic Viscosity of 0.54, as measured in 60/40Phenol/dichlorobenzene, was produced.

The above described CoPEN's were then coextruded through multi-layer diemanifolds to create a multi-layer film with 836 alternating first andsecond optical layers. This particular multi-layer reflective film alsocontains internal protective layers and external protective layerscomprised of the same copolyethylene naphthalate as the second opticallayers. This cast film was then uniaxially oriented at a 6:1 draw afterbeing heated to 163° C. to produce a reflective polarizer film ofapproximately 125 μm thickness.

When the described multi-layer reflective film was placed within an LCDcomputer display, the LCD display brightness increased by 56% whichcorrelates to a “Gain” of 1.56. Increases in LCD display brightness aremeasured as Gain, which was the ratio of the brightness of an LCDdisplay with brightness enhancing film to the brightness of an LCDdisplay without the brightness enhancing film. Typically, the displaybrightness was measured with an LS-100 or LS-110 luminance meter.Interlayer adhesion in the above described multi-layer reflective wasmeasured to be greater than 450 grams/inch (180 g/cm) using a standard90 degree tape peel test.

Example 11

A multi-layer reflective polarizer film was constructed with firstoptical layers created from polyethylenenaphthalate comprised of 100 mol% naphthalene dicarboxylate as the carboxylate, and 100 mol % ethyleneglycol as the diol. Second optical layers were created fromcopolyethylenenaphthalate comprised of 55 mol % naphthalenedicarboxylate and 45 mol % terephthalate as carboxylates, and 95.8 mol %ethylene glycol, 4 mol % hexane diol, and 0.2 mol % trimethylol propaneas glycols. Thwas particular multi-layer film also contained externalprotective layers created from copolyethylenenaphthalate comprised of 75mol % naphthalene dicarboxylate and 25 mol % terephthalate ascarboxylates, and 95.8 mol % ethylene glycol, 4 mol % hexane diol, and0.2 mol % trimethylol propane as glycols.

Polyethylenenaphthalate used to form the first optical layers wassynthesized in a batch reactor with the following raw material charge;136 kg dimethyl naphthalene dicarboxylate, 73 kg ethylene glycol, 27grams manganese acetate, 27 grams cobalt acetate, and 48 g antimonytri-acetate. Under pressure of 2 atm, this mixture was heated to 254° C.while removing the transesterification reaction by-product methanol.After 35 kg of methanol was removed, 49 g of triethyl phosphonoacetatewas charged to the reactor and than the pressure was gradually reducedto 1 torr while heating to 290° C. The condensation reaction by-product,ethylene glycol, was continuously removed until a polymer with anIntrinsic Viscosity of 0.48, as measured in 60/40phenol/dichlorobenzene, was produced.

Copolyethylenenaphthalate used to form the second optical layers wassynthesized in a batch reactor with the following raw material charge;88.5 kg dimethyl naphthalene dicarboxylate, 57.5 kg dimethylterephthalate, 81 kg ethylene glycol, 6.2 kg hexane diol, 29 gramscobalt acetate, 29 g zinc acetate, 239 g trimethylol propane, and 51 gantimony tri-acetate. Under pressure of 2 atm, this mixture was heatedto 254° C. while removing the transesterification reaction by-productmethanol. After 39.6 kg of methanol was removed, 56 g of triethylphosphonoacetate was charged to the reactor and than the pressure wasgradually reduced to 1 torr while heating to 290° C. The condensationreaction byproduct, ethylene glycol, was continuously stripped until apolymer with an Intrinsic Viscosity of 0.54, as measured in 60/40phenol/dichlorobenzene, was produced.

Copolyethylenenaphthalate used to form the external protective layerswas synthesized in a batch reactor with the following raw materialcharge; 114.8 kg dimethyl naphthalene dicarboxylate. 30.4 kg dimethylterephthalate, 75 kg ethylene glycol, 5.9 kg hexane diol, 29 gramscobalt acetate, 29 g zinc acetate, 200 g trimethylol propane, and 51 gantimony tri-acetate. Under pressure of 2 atm, this mixture was heatedto 254° C. while removing the transesterification reaction by-productmethanol. After 39.6 kg of methanol was removed, 56 g of triethylphosphonoacetate was charged to the reactor and than the pressure wasgradually reduced to 1 torr while heating to 290° C. The condensationreaction byproduct, ethylene glycol, was continuously stripped until apolymer with an Intrinsic Viscosity of 0.52, as measured in 60/40phenol/dichlorobenzene, was produced.

The above described CoPEN's were then coextruded through multi-layer diemanifolds to create a multi-layer film with 836 alternating first andsecond optical layers. This particular multi-layer reflective film alsocontains internal protective layers comprised of the same copolyethylenenaphthalate as the second optical layers. This cast film was thenuniaxially oriented at a 6:1 draw after being heated to 160° C. toproduce a reflective polarizer film of approximately 125 μm thickness.

When the described multi-layer reflective film was placed within an LCDcomputer display, the LCD display brightness increased by 58% whichcorrelates to a “Gain” of 1.58. Increases in LCD display brightness aremeasured as Gain, which was the ratio of the brightness of an LCDdisplay with brightness enhancing film to the brightness of an LCDdisplay without the brightness enhancing film. Typically, the displaybrightness was measured with an LS-100 or LS-110 luminance meter.

Interlayer adhesion in the above described multi-layer reflective wasmeasured to be greater than 450 grams/inch (180 g/cm) using a standard90 degree tape peel test.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the present specification. Theclaims are intended to cover such modifications and devices.

1. A multilayered polymer film, comprising: a plurality of first opticallayers comprising a birefringent first copolyester; and a plurality ofsecond optical layers comprising a second copolyester having an in-planebirefringence of about 0.04 or less, at 632.8 nm, after the multilayeredpolymer film has been formed, wherein the second copolyester comprisesglycol subunits derived from pentaerythritol.
 2. The multilayeredpolymer film of claim 1, wherein 0.01 to 5 mol % of the glycol subunitsof the second copolyester are derived from pentaerythritol.
 3. Themultilayered polymer film of claim 2, wherein 0.1 to 2.5 mol % of theglycol subunits of the second copolyester are derived frompentaerythritol.
 4. The multilayered polymer film of claim 1, whereinthe first copolyester comprises naphthalate subunits.
 5. Themultilayered polymer film of claim 1, wherein the second copolyestercomprises naphthalate subunits.
 6. The multilayered polymer film ofclaim 5, wherein the second copolyester further comprises terephthalatesubunits.
 7. The multilayered polymer film of claim 1, wherein thesecond copolyester further comprises carboxylate subunits derived fromcyclohexane dicarboxylic acid or an ester thereof.
 8. The multilayeredpolymer film of claim 1, wherein the second copolyester furthercomprises glycol subunits derived from C2–C4 diols.
 9. The multilayeredpolymer film of claim 8, wherein the second copolyester furthercomprises glycol subunits derived from 1,6-hexanediol or isomersthereof.
 10. The multilayered polymer film of claim 1, wherein onein-plane index of refraction of the first copolyester is approximatelyequal to one in-plane index of refraction of the second copolyesterafter the multilayered polymer film has been formed.
 11. Themultilayered polymer film of claim 1, wherein the first and secondcopolyesters both comprise naphthalate subunits.